In the field of data processing, and particularly in the field of data processing such as transmission, recording or reading out of predetermined data by way of predetermined media, there are processes wherein the data are first converted into a predetermined type of data format suitable for the medium to be used. The converted data are then input into and output from the medium, and thereafter, reconstituted into the original data. Such data processing occurs, for example in: power-line carrier communication of a variety of information including voice, image, etc.; communication using telephone lines as its medium; data transmission employing ADSL technology or other XDSL technologies using metallic wires as its medium; data transmission by way of wireless communication in the 2.4 GHz frequency band employing Bluetooth or wireless LAN; data transmission by way of coaxial cables for a CATV service employing CATV technology; employment of HomePNA (Home Phoneline Networking Alliances) using telephone wires present within each home; technologies using optical fiber as their medium; and wireless communication technologies in 800 MHz or 1.9 GHz frequency band when using cell phones or PHS phones, respectively. In the data recording and readout field, technologies to utilize magnetic disks or optical disks, etc., are included. Also, technologies that convert predetermined digital data into bar codes, print the bar codes and read them by using lasers or CCDs so as to reconstitute them into the original data, are included, as well as other equivalent technologies, in the applicable scope of the present invention.
In these data processing technologies, an apparatus that performs the steps of first converting or modulating predetermined data into a predetermined data format suitable for the medium to be used, then outputting the data from the medium and reconstituting to obtain the original data can be considered as a “modem” in the broad sense of the term. In the following examples, for explanatory convenience, descriptions are made in connection with a modem for power-line carrier communication, however, the applicable scope of the present invention is not limited to power-line carrier communication modems. In fact, the present invention is applicable to a variety of fields as stated above.
FIG. 1 shows such a power-line carrier communication system. In the drawing, a distribution substation is indicated by a reference numeral 101, as well as an access node 102, a high-voltage distribution line 103, a pole transformer 104, a low-voltage distribution line 105, a drop wire 106 and interior electric wiring 107.
In this system, high-voltage AC power (for example, 66 kilo VAC) is supplied from the distribution substation 101, by way of the high-voltage distribution line 103, to each pole transformer 104. Then it is stepped down, by means of the pole transformer 104, to obtain electric power of 100 volts or 200 volts that is suitable to be supplied to a consumer's house, such as each home, etc. Then the stepped-down power is supplied, by way of the low-voltage distribution line 105 and the drop wire 106, to the indoor electric wiring 107 of the consumer's house. Thus, various electric equipment connected to the indoor wiring 107 or various electric appliances of which power cords are plugged into convenience outlets can be powered and operated.
Additionally, the access node 102 located at the distribution substation 101 and a modem (not shown in this figure) located at the pole transformer 104 are connected to each other, with a fiber-optic transmission line (not shown in this figure). The fiber-optic transmission line is generally set along the route of the high-voltage distribution line 103. In the modem located at the pole transformer 104, bidirectional signal conversions of optical signals to electric signals and vice versa are carried out. The low-voltage distribution line 105, the drop wire 106 and the indoor wiring 107 are utilized as wire-link data transmission lines. Consequently, just a connection of a terminal unit to the wall socket connected to the indoor wiring 107 can configure a power-line carrier communication system, i.e., the system portion so-called “the last one mile” that can perform data transmission between the access node 102 and the terminal unit.
In such a power-line carrier communication system, the low-voltage distribution line 105, as seen from the modem located at the pole transformer 104, constitutes inductive impedance. In contrast, the drop wire 106 and the indoor wiring 107 constitute capacitive impedance. Further, each unit of the various electric equipment connected to the indoor wiring 107 generally has a structure connecting a capacitor for cutting off noise. Therefore, impedance of the low-voltage distribution line side seen from the modem located at the pole transformer 104 has relatively high inductance and capacitance.
As a result, the low-voltage distribution line (105) side, as seen from the modem located at the pole transformer 104, can be regarded as a low-pass filter, which means that the receive signals coming into another modem connected to the indoor wiring 107 have already been subjected to a substantial attenuation with respect to the high-frequency-range component thereof. Therefore, there is a possibility that the broadband component of the receive signals is submerged in the noise. The low-frequency-range component of the receive signals is not attenuated in such a degree as the high-frequency-range component is, but is quite vulnerable to random noise resulting from various electric equipment, such as an inverter circuit, a switching power supply, etc.